Electronically-controlled internal combustion engines available in recent years control an air-fuel ratio and the like on the basis of the output of an exhaust gas sensor placed in an exhaust pipe. Exhaust gas sensors generally exhibit poor sensing precision (or fail to function) before the sensor elements achieve activation temperatures. Hence, a heater inside an exhaust gas sensor is used to heat its sensor element after the start of the internal combustion engine in order to activate the exhaust gas sensor.
A known type of exhaust gas sensors (for example, NOX sensors) is provided with a sensor element including a plurality of cells. Systems are available that control the power to the heater such that an impedance (internal resistance) measured from one of the cells as temperature information reaches a target impedance that corresponds to the activation temperature of the cell measured.
The cells have different activation temperatures; thus, by controlling the power to the heater such that the impedance of only the cell measured reaches a target impedance, the temperature of another cell may not reach its activation temperature.
In Patent Literature 1, the power to the heater is controlled such that the resistance value of a cell to be measured achieves a first predetermined resistance value. Then, the power to the heater is further controlled such that the resistance value of the cell to be measured achieves a second predetermined resistance value that is greater than the first predetermined resistance value.